The present invention relates to an optical recording medium for recording and reading information by the use of a light beam such as a laser beam and, more particularly, to the format of such guide track on an optical recording medium that takes a spiral form.
An optical recording medium such as an optical disk is able to record information using a laser beam which is emitted by an optical head and which is focused to a spot of the order of 1 .mu.m in diameter. The information can be read by a read/write head that is placed in noncontacting relation to the disk. Accordingly, it has attracted interest for high-density, digital mass storage.
A guide track in the form of a groove has been previously formed on an optical disk, and the neighboring turns, of this track are spaced apart a distance on the order of microns. A layer to be recorded is pierced or the reflectivity of the layer is changed by the energy of a laser beam having a minute diameter while the guide track is being traced by the thin laser beam, in order to record information.
Optical disks for recording digital data have a guide track which is divided into a plurality of sectors in accordance with a certain format, like magnetic disks. Thus, a user can select a desired one of the sectors at will to record and read information. This format is formed at the same time when the guide track is cut by a laser cutting machine on a master disk coated with photoresist. The obtained master disk is subjected to development, and then it is plated with nickel. The plating layer of nickel is peeled off from the body of the master disk to provide a master disk of nickel on which the images that were formed on the photoresist layer by laser cutting have been transferred. This master disk of nickel is then set in a plastic mold, into which a plastic material is injected to form a replica disk that stores information formatted as described above. A recording material is then deposited onto the replica disk to form a recording layer. Subsequently, a protective layer is formed on this recording layer, thus completing the optical disk.
One example of this format of the optical, disk is disclosed in U.S. Pat. Ser. No. 444,456 now U.S. Pat. No. 4,535,439, entitled "Optical Having Index Mark and Sector Mark", filed Nov. 24, 1982, in which the disk is divided into a plurality of sectors equally spaced apart from each other. The sectors are provided with a sector separator of identical structure at their foremost positions. Address parts which store addresses for the turns of the track are formed in the specific portion of the disk,i.e., the region extending radially from the center of rotation of the disk. Those which are cut in the innermost region of the disk are first and second index parts corresponding to the address parts and the sector separators, respectively. The first and second index parts are detected as changes in the quantity of reflected light by specially installed index detecting devices. The first index parts are used to determine the timing at which the light beam jumps back to its starting point after completing one trace of a specific portion of the spiral guide track, for retracing purposes. The second index parts are counted by a counter which delivers an output signal to indicate sector addresses for the sectors on the optical disk. The sector separators and the first and second index parts are shaped into depressions according to signals in the form of a burst.
The format as described thus far has the following features. Since the optical head is caused to jump in response to the detection of one first index part, it is hardly affected by drop-out occurring on the optical disk. This ensures that the head is made to jump with certainty. Further, the sector addresses are read at a low error rate. However, special devices are needed to detect the first and second index parts. In addition, it is difficult to correctly adjust the positions of these detecting devices relative to the read/write head. If some sector separators cannot be detected, the corresponding sector addresses will be judged to be absent.